Robert C. Wills

Thermodynamic and dynamic controls on changes in the zonally anomalous hydrological cycle

The wet gets wetter, dry gets drier paradigm explains the expected moistening of the extratropics and drying of the subtropics as the atmospheric moisture content increases with global warming. Here we show, using precipitation minus evaporation (P − E) data from climate models, that it cannot be extended to apply regionally to deviations from the zonal mean. Wet and dry zones shift substantially in response to shifts in the stationary-eddy circulations that cause them. Additionally, atmospheric circulation changes lead to a smaller increase in the zonal variance of P − E than would be expected from atmospheric moistening alone. The P − E variance change can be split into dynamic and thermodynamic components through an analysis of the atmospheric moisture budget. This reveals that a weakening of stationary-eddy circulations and changes in the zonal variation of transient-eddy moisture fluxes moderate the strengthening of the zonally anomalous hydrological cycle with global warming.

Stationary Eddies and the Zonal Asymmetry of Net Precipitation and Ocean Freshwater Forcing

Transport of water vapor in the atmosphere generates substantial spatial variability of net precipitation n(precipitation minus evaporation). Over half of the total spatial variability in annual-mean net precipitation is naccounted for by deviations from the zonal mean. Over land, these regional differences determine differences in nsurface water availability. Over oceans, they account, for example, for the Pacific–Atlantic difference in sea nsurface salinity, with implications for the deep overturning circulation. This study analyzes the atmospheric-water nbudget in reanalyses from ERA-Interim and MERRA, to investigate which physical balances lead to zonal nvariation in net precipitation. It is found that the leading-order contribution is zonal variation in stationary-eddy nvertical motion. Transient eddies modify the pattern of zonally anomalous net precipitation by moving moisture nfrom the subtropical and tropical oceans onto land and poleward across the Northern Hemisphere storm tracks. nZonal variation in specific humidity and stationary-eddy horizontal advection play a secondary role. The dynamics nleading to net precipitation via vertical motion in stationary eddies can be understood from a lower-tropospheric nvorticity budget. The large-scale variations of vertical motion are primarily described by Sverdrup nbalance and Ekman pumping, with some modification by transient eddies. These results suggest that it is important nto understand changes in stationary eddies and their influence on the zonal variation of transient eddy nfluxes, in order to understand regional changes in net precipitation. They highlight the relative importance of ndifferent atmospheric mechanisms for the freshwater forcing of the North Pacific and North Atlantic.

How Stationary Eddies Shape Changes in the Hydrological Cycle: Zonally Asymmetric Experiments in an Idealized GCM

Stationary and low-frequency Rossby waves are the primary drivers of extratropical weather variations on monthly and longer time scales. They take the form of persistent highs and lows, which, for example, shape subtropical dry zones and guide extratropical storms. More generally, stationary-eddy circulations, including zonally anomalous tropical overturning circulations, set up large zonal variations in net precipitation (precipitation minus evaporation, P − E). This paper investigates the response of stationary eddies and the zonally asymmetric hydrological cycle to global warming in an idealized GCM, simulating a wide range of climates by varying longwave absorption. The stationary eddies are forced by two idealized zonal asymmetries: a midlatitude Gaussian mountain and an equatorial ocean heat source. Associated with changes in stationary eddies are changes in the zonal variation of the hydrological cycle. Particularly in the subtropics, these simulations show a nearly constant or decreasing amplitude of the zonally anomalous hydrological cycle in climates warmer than modern despite the wet gets wetter, dry gets drier effect associated with increasing atmospheric moisture content. An approximation for zonally anomalous P − E, based on zonal-mean surface specific humidity and stationary-eddy vertical motion, disentangles the roles of thermodynamic and dynamic changes. The approximation shows that changes in the zonally asymmetric hydrological cycle are predominantly controlled by changes in lower-tropospheric vertical motion in stationary eddies.

Disentangling Global Warming, Multidecadal Variability, and El Niño in Pacific Temperatures

A key challenge in climate science is to separate observed temperature changes into components due to internal variability and responses to external forcing. Extended integrations of forced and unforced climate models are often used for this purpose. Here we demonstrate a novel method to separate modes of internal variability from global warming based on differences in time scale and spatial pattern, without relying on climate models. We identify uncorrelated components of Pacific sea surface temperature variability due to global warming, the Pacific Decadal Oscillation (PDO), and the El Nino–Southern Oscillation (ENSO). Our results give statistical representations of PDO and ENSO that are consistent with their being separate processes, operating on different time scales, but are otherwise consistent with canonical definitions. We isolate the multidecadal variability of the PDO and find that it is confined to midlatitudes; tropical sea surface temperatures and their teleconnections mix in higher‐frequency variability. This implies that midlatitude PDO anomalies are more persistent than previously thought.

Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean

The interplay between ocean circulation and biological productivity affects atmospheric CO2 levels and marine oxygen concentrations. During the warming of the last deglaciation, the North Pacific experienced a peak in productivity and widespread hypoxia, with changes in circulation, iron supply and light limitation all proposed as potential drivers. Here we use the boron-isotope composition of planktic foraminifera from a sediment core in the western North Pacific to reconstruct pH and dissolved CO2 concentrations from 24,000 to 8,000 years ago. We find that the productivity peak during the Bølling–Allerød warm interval, 14,700 to 12,900 years ago, was associated with a decrease in near-surface pH and an increase in pCO2, and must therefore have been driven by increased supply of nutrient- and CO2-rich waters. In a climate model ensemble (PMIP3), the presence of large ice sheets over North America results in high rates of wind-driven upwelling within the subpolar North Pacific. We suggest that this process, combined with collapse of North Pacific Intermediate Water formation at the onset of the Bølling–Allerød, led to high rates of upwelling of water rich in nutrients and CO2, and supported the peak in productivity. The respiration of this organic matter, along with poor ventilation, probably caused the regional hypoxia. We suggest that CO2 outgassing from the North Pacific helped to maintain high atmospheric CO2 concentrations during the Bølling–Allerød and contributed to the deglacial CO2 rise.The upwelling of carbon- and nutrient-rich waters in the subpolar North Pacific during the Bølling–Allerød supported high productivity and CO2 outgassing, as well as contributing to regional hypoxia, marine sediment analyses suggest.

Local Energetic Constraints on Walker Circulation Strength

The weakening of tropical overturning circulations is a robust response to global warming in climate models and observations. However, there remain open questions on the causes of this change and the extent to which this weakening affects individual circulation features such as the Walker circulation. The study presents idealized GCM simulations of a Walker circulation forced by prescribed ocean heat flux convergence in a slab ocean, where the longwave opacity of the atmosphere is varied to simulate a wide range of climates. The weakening of the Walker circulation with warming results from an increase in gross moist stability (GMS), a measure of the tropospheric moist static energy (MSE) stratification, which provides an effective static stability for tropical circulations. Baroclinic mode theory is used to determine changes in GMS in terms of the tropical-mean profiles of temperature and MSE. The GMS increases with warming, owing primarily to the rise in tropopause height, decreasing the sensitivity of the Walker circulation to zonally anomalous net energy input. In the absence of large changes in net energy input, this results in a rapid weakening of the Walker circulation with global warming.

Mechanisms Setting the Strength of Orographic Rossby Waves across a Wide Range of Climates in a Moist Idealized GCM

AbstractOrographic stationary Rossby waves are an important influence on the large-scale circulation of the atmosphere, especially in Northern Hemisphere winter. Changes in stationary waves with gl...